• Sublime Core Feed
Medium Severity

Credential phishing: Fake storage alerts (unsolicited)

Description

This rule targets credential phishing attempts disguised as storage space alerts, activating for inbound emails with specific storage-related keywords and evaluating sender trustworthiness and history.

References

No references.

Sublime Security
Created Mar 6th, 2024 • Last updated Jan 12th, 2026
Source
type.inbound
and (
  (
    0 < length(body.links) < 8
    and any([subject.subject, sender.display_name],
            regex.icontains(., "storage|mailbox")
    )
  )
  or (
    //
    // This rule makes use of a beta feature and is subject to change without notice
    // using the beta feature in custom rules is not suggested until it has been formally released
    //
    any(ml.nlu_classifier(beta.ocr(file.message_screenshot()).text).intents,
        .name == "cred_theft" and .confidence == "high"
    )
    and regex.icontains(beta.ocr(file.message_screenshot()).text,
                        "storage.{0,50}full",
                        "free.{0,50}upgrade",
                        "storage.{0,50}details",
                        "storage.{0,50}quot",
                        "email.{0,50}storage",
                        "total.{0,50}storage",
                        "storage.{0,50}limit"
    )
    and not strings.ilike(beta.ocr(file.message_screenshot()).text,
                          "*free plan*"
    )
  )
  or (
    any(body.links,
        // fingerprints of a hyperlinked image
        .display_text is null
        and .display_url.url is null
        and (
          .href_url.domain.root_domain in $free_file_hosts
          or .href_url.domain.root_domain == "beehiiv.com"
        )
    )
    and length(attachments) == 1
    and all(attachments,
            .file_type in $file_types_images
            and .size > 2000
            and any(file.explode(.),
                    regex.icontains(.scan.ocr.raw,
                                    "storage.{0,50}full",
                                    "free.{0,50}upgrade",
                                    "storage.{0,50}details",
                                    "storage.{0,50}quot",
                                    "email.{0,50}storage",
                                    "total.{0,50}storage"
                    )
            )
    )
  )
)
and (
  regex.icontains(subject.subject, '\bfull\b')
  or strings.icontains(subject.subject, "exceeded")
  or strings.icontains(subject.subject, "out of")
  or strings.icontains(subject.subject, "mailbox")
  or strings.icontains(subject.subject, "icloud")
  or regex.icontains(subject.subject, '\blimit(?:ed|\b)')
  or strings.icontains(subject.subject, "all storage used")
  or strings.icontains(subject.subject, "compliance")
  or strings.icontains(subject.subject, "max storage")
  or strings.icontains(subject.subject, "storage space")
  or strings.icontains(subject.subject, "be deleted")
  or strings.icontains(subject.subject, "action required")
  or strings.icontains(subject.subject, "undelivered messages")
  or strings.icontains(subject.subject, "review storage")
  or regex.icontains(subject.subject, "upgrade (today|now)")
)

// negate customer service requests about storage
and not any(ml.nlu_classifier(body.current_thread.text).topics,
            .name == "Customer Service and Support" and .confidence == "high"
)

// negate links to loopnet.com - a popular commerical property listing service
and not (any(body.links, .href_url.domain.root_domain == "loopnet.com"))

// negate legitimate sharepoint storage alerts
and (
  (
    sender.email.email == "no-reply@sharepointonline.com"
    and not headers.auth_summary.dmarc.pass
    and (
      not all(body.links,
              .href_url.domain.root_domain in~ (
                "sharepoint.com",
                "microsoft.com",
                "aka.ms"
              )
      )
    )
  )
  or sender.email.email != "no-reply@sharepointonline.com"
)

// negate legitimate iCloud China storage alerts
and (
  (
    sender.email.email == "noreply@icloud.com.cn"
    and not headers.auth_summary.dmarc.pass
    and (
      not all(body.links,
              .href_url.domain.root_domain in~ ("icloud.com", "aka.ms")
      )
    )
  )
  or sender.email.email != "noreply@icloud.com.cn"
)

// negate bouncebacks and undeliverables
and not any(attachments,
            .content_type in (
              "message/global-delivery-status",
              "message/delivery-status",
            )
            or (
              .content_type == "message/rfc822"
              and any(file.parse_eml(.).attachments,
                      .content_type in (
                        "message/global-delivery-status",
                        "message/delivery-status",
                      )
              )
            )
)

// negate highly trusted sender domains unless they fail DMARC authentication
and (
  (
    sender.email.domain.root_domain in $high_trust_sender_root_domains
    and not headers.auth_summary.dmarc.pass
  )
  or sender.email.domain.root_domain not in $high_trust_sender_root_domains
)
and (
  not profile.by_sender().solicited
  or profile.by_sender().any_messages_malicious_or_spam
)
// negate instances where proofpoint sends a review of a reported message via analyzer 
and not (
  sender.email.email == "analyzer@analyzer.securityeducation.com"
  and any(headers.domains, .root_domain == "pphosted.com")
  and headers.auth_summary.spf.pass
  and headers.auth_summary.dmarc.pass
)
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